Using in situ capacitance measurements to monitor the stability of interface materials in complex pcb assemblies and other structures

ABSTRACT

An electric potential is applied to first and second electrodes on opposite sides of a gap between an electronic component and a heat spreader. At least one of a thermal interface material in the gap, the electronic component and the heat spreader is subjected to a changing physical condition. The capacitance is monitored. Such a method can be practiced using an array of components sharing a common heat spreader. An assembly for testing thermal interfaces includes a printed circuit board, a plurality of electronic components mounted to and operatively associated with the printed circuit board, a heat spreader positioned for absorbing heat generated by the electronic components, a first electrode associated with the heat spreader, a plurality of second electrodes associated, respectively, with the electronic component, and a device for monitoring electrical capacitances. The technique may be employed for monitoring physical changes in electronic devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 14/082,531 filed Nov. 18, 2013, which in turn is a divisional ofU.S. patent application Ser. No. 13/010,854 filed 01-21-2011, now U.S.Pat. No. 8,589,102, which in turn claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/377,281 filed on Aug. 26, 2010. Thecomplete disclosures of the aforementioned U.S. patent application Ser.Nos. 14/082,531 and 13/010,854 and Provisional Patent Application Ser.No. 61/377,281 are expressly incorporated herein by reference in theirentireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to the electronic, thermal, and mechanicalarts, and, more particularly, to thermal control and monitoring ofelectronic modules and other structures.

BACKGROUND OF THE INVENTION

Thermal interface materials (“TIM”) are employed to fill the gapsbetween thermal transfer surfaces. For example, such materials can fillthe gaps between electronic components and heat sinks. They allow theconduction of heat from the components to the heat sinks. TIM can bemade from various materials. TIM properties can be different dependingon their composition.

SUMMARY OF THE INVENTION

Principles of the invention provide techniques for using in situcapacitance measurements to monitor the stability of materialsexhibiting dielectric properties, such as certain thermal interfacematerials, in structures such as complex PCB (printed circuit board)assemblies. In one aspect, an exemplary method includes the steps ofapplying an electric potential to first and second electrodes onopposite sides of a gap between an electronic component and a heatspreader, subjecting at least one of a thermal interface material in thegap, the electronic component and the heat spreader to a changingphysical condition, and monitoring the electrical capacitance betweenthe electrodes during the changing physical condition. Such a method canbe practiced using an array of components sharing a common heatspreader.

In another aspect, an exemplary assembly for testing thermal interfacesincludes a printed circuit board; a plurality of electronic componentsmounted to and operatively associated with the printed circuit board; aheat spreader positioned for absorbing heat generated by the electroniccomponents; a first electrode associated with the heat spreader; aplurality of second electrodes associated, respectively, with theelectronic components; and a device for monitoring electricalcapacitances between the first and second electrodes.

In a further aspect, an exemplary method of constructing an assemblycapable of being monitored for physical changes includes the steps ofproviding a plurality of components, providing interface material,providing a plurality of electrode plates, providing a plurality ofconductors, and assembling the components, the interface material, theelectrode plates and the conductors to form an integral structurewherein a plurality of interfaces are provided between the components,the electrode plates are positioned within the interfaces, the interfacematerial is positioned between the electrode plates, and the conductorsare electrically connected to the electrode plates and are externallyaccessible on the integral structure. The method may further includeproviding an apparatus for measuring electrical capacitance andconnecting the apparatus to the conductors.

As used herein, “facilitating” an action includes: performing theaction, making the action easier, helping to carry the action out, orcausing the action to be performed. Thus, by way of example and notlimitation, instructions executing on one processor might facilitate anaction carried out by instructions executing on a remote processor, bysending appropriate data or commands to cause or aid the action to beperformed. For the avoidance of doubt, where an actor facilitates anaction by other than performing the action, the action is neverthelessperformed by some entity or combination of entities.

One or more embodiments of the invention or elements thereof can beimplemented in the form of a computer product including a tangiblecomputer readable recordable storage medium with computer usable programcode for performing the method steps indicated. Furthermore, one or moreembodiments of the invention or elements thereof can be implemented inthe form of a system (or apparatus) including a memory, and at least oneprocessor that is coupled to the memory and operative to performexemplary method steps. Yet further, in another aspect, one or moreembodiments of the invention or elements thereof can be implemented inthe form of means for carrying out one or more of the method stepsdescribed herein; the means can include (i) hardware module(s), (ii)software module(s), or (iii) a combination of hardware and softwaremodules; any of (i)-(iii) implement the specific techniques set forthherein, and the software modules are stored in a tangiblecomputer-readable recordable storage medium (or multiple such media).

Techniques of the present invention can provide substantial beneficialtechnical effects. For example, one or more embodiments may provide oneor more of the following advantages:

-   -   The air gap in which the thermal interface material or other        material will reside can be accurately measured before        depositing the material, which allows knowing the ultimate bond        line for the material. These air gap measurements can also be        compared to specification targets with tolerances for the        material to ensure, for example, that a heat spreader and        electronic assembly meet their respective design targets for        dimensions such as x, y and z location;    -   The bond line can be measured in real time during the attachment        of one element to another, for example the attachment of a heat        spreader to an electronic assembly. Processes to attach the heat        spreader can accordingly be developed and optimized to ensure        that the target TIM gap is reached;    -   Average thermal performance can be predicted for a complex        electronic assembly before designing and building special        thermal functional test hardware. This capability allows        detection of time zero design deficiencies as well as        environmental stress test driven degradation that would result        in unreliable field performance;    -   Gap motion can be measured as a function of temperature, which        allows the design of thermal mechanical experiments on model        samples to evaluate mechanical stability of TIMs and the impact        of surface treatments and mechanical constraints on TIM        stability.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of a TIM bond line response to load, time andfastening of a heat spreader to a printed circuit board;

FIG. 2 shows a graph illustrating TIM bond line response to thermalcycling of a selected electronic assembly;

FIG. 3 shows a graph illustrating TIM bond line response of anelectronic component in response to thermal cycling with peak to valleymotion less than 0.05 mm;

FIG. 4 shows photographs of residual TIM on a heat spreader location(left) and an electronic component (right) following thermal cycletesting;

FIG. 5 shows a graph illustrating TIM bond line response of anelectronic component in response to thermal cycling suggesting peak tovalley motion of about 0.4 mm;

FIG. 6 shows photographs of a heat spreader location (left) and anelectronic component (right) following thermal cycle testing and showingasymmetric fillets and about a 25% loss in TIM area;

FIG. 7 shows a graph illustrating absolute air gap motion for aplurality of electronic components during a thermal cycle from −40 to60° C.;

FIG. 8 shows a graph illustrating distribution of strain in air gaps forthirty-two electronic components;

FIG. 9 shows photographs of a heat spreader location (top left), anelectronic component (bottom left) following a thermal cycle test, and agraph illustrating the gap between the electronic component and the heatspreader as a function of thermal cycles;

FIG. 10 shows photographs of a heat spreader location (top left) and anelectronic component (bottom left) following a thermal cycle test, and agraph illustrating the gap between the electronic component and a heatspreader as a function of thermal cycles;

FIG. 11 shows a graph illustrating correlation between strain and meanshift in capacitive bond line;

FIG. 12 shows photographs of a smooth heat spreader location (left) andthe bottom of an electronic component (right) after power cycle testingand not showing evidence of TIM pumping;

FIG. 13 shows photographs of a smooth heat spreader location (left) andthe bottom of an electronic component (right) after power cycle testingand showing evidence of TIM pumping;

FIG. 14 shows photographs of a smooth heat spreader location (left) andan electronic component (right) after thermal cycle testing withoutusing a copper electrode, the photographs showing evidence of TIMpumping including about a twenty percent loss in TIM area;

FIG. 15 shows photographs of a heat spreader location (left) and anelectronic component after a thermal cycle test without using a copperelectrode and wherein the heat spreader has a surface roughened to N9,no evidence of TIM pumping being present;

FIG. 16 shows photographs of a heat spreader location (left) and anelectronic component (right) after power cycle testing without using acopper electrode, the heat spreader having a surface roughened to N9, noevidence of TIM pumping being present;

FIG. 17 shows a graph indicating capacitive gap measurements duringpower cycling, the gap being with and without TIM;

FIG. 18 shows a graph illustrating TIM gap motion during power andtemperature cycling;

FIG. 19A is a schematic diagram showing an assembly for determining thecapacitance of TIM in a gap between an electronic component and a heatspreader;

FIG. 19B is an enlarged perspective view of an electronic component andassociated copper and dielectric layers;

FIG. 19C is a schematic diagram showing the assembly of FIG. 19A whereinthe heat spreader is configured to function as an electrical shield;

FIG. 20 is a schematic diagram showing an assembly for monitoring thecapacitance of a plurality of electronic components in a device undertest;

FIG. 21 is a flow chart of exemplary method steps, according to anaspect of the invention; and

FIG. 22 depicts a computer system that may be useful in implementing oneor more aspects and/or elements of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Aspects of the invention pertain to monitoring thermal interfacematerials used in conjunction with electronic components such as modulesand/or other electronic devices. As noted above, thermal interfacematerials (“TIM”) are employed to fill the gaps between thermal transfersurfaces. Materials used to fill the gaps between electronic componentsand heat sinks often include polymeric materials filled with thermallyconductive fillers. The polymeric materials may include greases, gels,waxes, elastomers, and rigid thermosets and thermoplastics. Thermallyconductive fillers often include silica, alumina, boron nitride,aluminum nitride, aluminum, or zinc oxide. Pre-cured, room temperaturecurable, thermal or light curable, or non-curing TIMs are available. Themethods described herein are applicable to materials having a measurabledielectric constant. Accordingly, electrically conductive fillers couldbe present in the materials as long as the materials continue to supportan electrostatic field (for example, under circumstances where fillercontent is below the percolation threshold where isotropic electricalconductivity is not possible). Electrically conductive fillers found insome TIMs include silver, gold, copper and nickel. TIMs allow theconduction of heat from the components to the heat sinks. TIM propertiescan be different depending on their composition. An embodiment of theinvention provides a thermal solution for an array of electroniccomponents, namely voltage transformer modules. The modules are arrangedto be cooled by a large area, common aluminum heat spreader for a highend server. An in-situ, capacitive bond line thermal measurementtechnique is used to measure the capacitance of a non-electricallyconducting thermal interface material (TIM) between one or more of theelectronic modules and the heat spreader to quantify the TIM bond lineeffective thickness during assembly, testing and operation. The thermalresistance of the TIM has the same geometric dependence as the inverseof capacitance. Accordingly, the capacitive technique also allows themonitoring of the thermal performance of the interface. It will beappreciated that the disclosed technique has possible application to awide variety of electronic components that are thermally interfaced toheat sinks by TIM, whether employed in servers or other electronicapparatus.

The capacitive technique was applied to measure the bond line in realtime during the assembly of the heat spreader to an array ofthirty-seven modules mounted on a printed circuit board (PCB). Theresults showed that the target bond lines were not achieved byapplication of a constant force alone on the heat spreader and guided animproved assembly process.

The mechanical motion of the TIM was monitored in-situ during severalhundred thermal cycles and found to fluctuate systematically from thehot to cold portions of the thermal cycle, either compressing orstretching the TIM, respectively. The capacitive bond line trend showedthermal interface degradation vs. cycle count for several modules, whichwas confirmed by disassembly and visual inspection. Areas of depletedTIM ranged as high as 25% of the component area.

Several design and material changes were shown to improve the TIMstability. Such improvements were detected through performance of thecapacitance technique, as described further below, and confirmed byvisual inspection. Power cycling tests were run in parallel to thermalcycle tests to help relate the results to field performance. Thecapacitance technique enabled the development and verification of athermal solution for a complex mechanical system very early in thedevelopment cycle.

Heat sinks have been individually attached to a variety of components oncomplex printed board assemblies. As packaging integration increases,more design forethought is required to ensure efficient and effectiveremoval of heat from high power electronic modules. More control overair flow is possible when a common heat spreader is interfaced to thevarious high heat generating modules. Air flow can be directed downchannels that are integrated into the heat spreader. There are certainchallenges when using a common heat spreader. A large area heat spreaderthat is thermally interfaced to many components will require greaterforce to reach the target bond line of the thermal interface material(TIM.) The several components, which could be the same design or avariety of designs, may have different heights above the printed circuitboard (PCB.) The TIM should easily accommodate tolerances in heightwhich typically could be +/−0.5 mm for a nominal gap of 1.5 mm.Similarly, it should be easy to separate the heat spreader afterwards incase one or more components fail electrical testing and requirereplacement. During the removal of the heat spreader, it is preferablethat the separation forces be low enough to prevent electrically goodcomponents from being damaged. Finally, TIM structural integrity istypically required across all components to provide a reliable thermalpath for heat transfer during the life cycle of the product.

Voltage transformer modules 102 intended for use in a high end serverare employed in experimental performance of the capacitance techniquedisclosed herein. FIG. 19A shows an assembly 100 for performing thetechnique. As discussed below, shielding of the wires and electroniccomponents as shown in the figure is preferred though not required forperforming the technique. Thirty-seven of these modules are surfacemounted to a large printed circuit board (PCB) 104 that is 380×760 mm. Acommon heat spreader 106 of similar dimensions is mated to the array ofmodules 102 and secured with several screws to fixed height standoffs(not shown). The voltage transformer module is sensitive to excessivemechanical compressive and tensile forces. In order to stay below theselimited force levels, a very low modulus silicone gel TIM 108 wasselected. This TIM easily accommodates the target nominal bond line of1.5 mm, +/−0.5 mm. Separation forces were <0.03 MPa. The thermalresistance of 675 C mm²/W at a nominal bond line was adequate for theapplication. There are other TIMs with lower thermal resistance (<350 Cmm²/W) that are still reworkable; however, the tensile separation forcesexceeded the fragile limit allowed for the particular componentsemployed in this embodiment. Initially, thermal pad TIMs were consideredbut, the compressive forces required to achieve the range of bond linesexceeded the fragile limit allowed for the component. The proposedcapacitance technique could be used in applications where thermal padTIMs are suitable for the particular component(s). It could also be usedin applications where individual heat sinks are employed for eachcomponent, though greater benefit is likely obtained when used inassemblies including a heat spreader associated with multiple electroniccomponents.

Studies and experiments were defined to measure and confirm that thebond lines met specifications, that the compressive and tensile forcesdid not exceed limits, and that the TIM remained in place during thermaland power cycling.

For non-electrically conducting TIMs, electrical capacitance between thecomponent and the heat spreader can provide a direct probe of the bondline thickness and material integrity of the TIM. In this work,capacitance measurements were made of the thermal interface at allprocess stages, from initial squeeze out, to final bond line and duringthermal mechanical stress testing. This technique provides a useful viewof a critical part of the thermal path and exposes the effects ofmechanical changes in the overall package that dramatically affect theTIM but can be nearly undetectable from outside the package.

Theory

Simplistically, the module and heat spreader with TIM in between istreated as a parallel plate capacitor. For a pair of parallel conductiveplates of area, A, spaced a distance, g, apart, the capacitance, C, isgiven by:

$\begin{matrix}{{C = \frac{ɛ_{r}ɛ_{0}A}{g}},} & \left. 1 \right)\end{matrix}$

where ∈₀=8.85e-12 Farads/meter is the permittivity of free space and∈_(r) is the relative dielectric constant of the material between theplates. The inverse dependence on g makes capacitance a very useful andsensitive detector for gap measurements, especially at small gaps.Measuring the capacitance for parallel plates with a known spacingfilled with the TIM material of interest allows one to determine itsdielectric constant by solving equation 1. Subsequently, capacitancemeasurements can be made and used to calculate an average, effectivebond line.

In certain circumstances, thermal conductance and capacitance have verysimilar geometrical and material property dependencies, makingcapacitance particularly relevant as an indicator of thermalperformance. In uniform heat flow, the conductance, G, between parallelsurfaces with a gap, g, filled with a material of conductivity k, andhaving an area, A, is given by:

$\begin{matrix}{G = \frac{kA}{g}} & \left. 2 \right)\end{matrix}$

Comparing equations 1 and 2, one can see that capacitance and thermalconductance both scale as area divided by gap, with dielectric constantplaying the same role as material thermal conductivity.

In electronic assemblies, the component and heat spreader thermalconductivities are typically much higher than that of the TIM, but notinfinite, so lateral gradients can be significant and the analogy to theelectrical situation will be imperfect. Therefore, capacitance typicallycannot be used as a substitute for detailed thermal modeling andmeasurement of packages. However, overall component to heat spreadercapacitance does reflect the average thermal conductance of the TIMmaterial and will track variations in thermal performance of parts. Themeasurement does not require special thermal test chips, so either earlydevelopment, mechanically good hardware or standard production parts canbe monitored. In addition, capacitance can be measured much more quicklyand with higher precision than thermal resistance. This allows largenumbers of parts to be studied, which reduces statistical uncertainty indesign and process evaluation experiments. The theory behind theproposed capacitance technique is provided herein solely for explanatorypurposes. The proposed technique has been found to be an effective toolregardless of theoretical assumptions.

Experimental

Several evolutionary experiments were completed that characterized theresponse of the design gap for the TIM at every step from initial heatspreader mating to the structural integrity of the TIM under cyclic,thermal mechanical strain. These experiments are described and discussedin three phases. Every phase provided insights that helped define thenext set of experiments. Phase 1 used one assembly to quantify how theTIM gaps develop during heat spreader mating. Also, testing provided anearly development look at how the TIM responded to cyclic, thermalmechanical strain induced during thermal cycling before thermal hardwarewas available. Phase 2 used ten (10) assemblies and compared TIM bondlines to design targets. The effectiveness of design changes thatresulted from Phase 1 learning was studied in depth. In Phase 3experiments, more realistic power cycling was performed and the impactof surfaces introduced as electrodes to make the capacitive bond linemeasurements was evaluated.

Phase 1 Experiments

The first experiment was to determine how the bond line developed as afunction of mating force, time and engagement of mechanical fasteners.Twenty of the thirty-seven components (modules) 102 were wired to makethe capacitance measurement. A piece of copper tape with electricallyinsulating adhesive was applied to the entire surface of each of thetwenty components. (The copper tape employed during the Phase 2experiments discussed below included two copper layers 110,112 separatedby a dielectric tape 114 as shown in FIGS. 19A and 19B.) The copper tapeor layer adjoining the thermal interface material is used as anelectrode of the capacitor. Wires 118 were soldered to areas ofoverhanging copper tape and carefully routed across the PCB surface,avoiding fastening standoffs and flow channel ribs in the heat spreader.The other ends of the wires were connected to a scanner switch box 120that was connected to an LCR meter 122 set at 10 KHz and 1 volt drive.The second electrode was the heat spreader 106 and this was connecteddirectly to the LCR meter. FIG. 20 provides an additional schematicillustration of the testing assembly showing further details of thescanner switch box 120 as well as the shielding 116 for the wires 118added for the Phase 2 experiments. Six switches are shown for purposesof illustration in the scanner switch box, though more or less may beemployed depending on the number of components 102 to be tested. Asindicated above, twenty of the thirty-seven components were tested inthe procedure and twenty switches were accordingly employed. Dataacquisition was controlled by a personal computer 130 using BASICprogramming and GPIB (General Purpose Interface Bus) 124. GPIB is onenon-limiting example of suitable cabling; for example, USB (universalserial bus) or the like could be employed in other instances. After allthe wiring and data logging connections were completed and the heatspreader was placed and secured, the parallel sum of stray and air gapcapacitances was measured at each of the twenty test sites. There wasstray capacitance from the voltage transformer module and PCB to thecopper tape electrode on the module and from the heat spreader to thewires running from the copper tape electrode to the switch box. Anaverage capacitance, C_(ave), was determined for each site (40.00 to65.00 pF, +/−0.03). Next, the heat spreader was removed, TIM 108deposited and the heat spreader 106 was reattached. The sum of theparallel capacitances, stray and TIM, C₂, was measured. The straycapacitance was removed by subtracting the average air gap/straycapacitance from the measured TIM gap/stray capacitance. Assuming thatthe air and TIM gap are equal, the gap with the TIM 108 was calculated.

$\begin{matrix}{g_{TIM} = \frac{ɛ_{0}{A\left( {ɛ_{r} - 1} \right)}}{C_{2} - C_{ave}}} & \left. 3 \right)\end{matrix}$

Capacitance, temperature and time were logged continuously. The matingforce applied to the heat spreader was limited to 250 kg which wasdetermined by assuming a uniform loading of all thirty-seven modules to90% of the compressive limit. FIG. 1 is a plot of the TIM bond line vs.time and mechanical loading. The ultimate bond lines were not achievedwith 250 kg mating force and time alone. Only after engaging the screwsand securing to the standoffs were stable bond lines achieved, rangingbetween 1.2 to 1.7 mm.

Instantaneous loads can be very high when screws are engaged given thetypical strain rate dependency of the TIM with respect to stiffnessduring compression. In order to ensure that the individual compressiveloads on voltage converter modules were below the damage level, in-situforce measurements were made. A screw fastening sequence and rate weredefined to keep the instantaneous loading on any single component in asafe region. The 250 kg loading was dropped from the heat spreaderattachment process. Several of the bond lines were also measured byleaving out the TIM and placing a sandwich of room temperature curingepoxy between thin release layers, on the component surface. The heatspreader was placed when uncured epoxy was in place and it was removedafter the epoxy cured. A precision micrometer was used to measure thecured disk of epoxy and the agreement with the bond line calculated fromcapacitance was within 10%. One difference in the two measurementmethods was that the bond line calculated from capacitance representedthe entire area of the component while the epoxy disks covered less than10% of the area in the center.

The next experiment was to monitor the integrity of the TIM in theassembly during thermal cycling. TIM was applied on the heat spreader toall thirty-seven component locations. After the heat spreader was matedand secured with screws, the capacitance was measured ten times on thebench to ensure that electrical connections were good and that thecapacitance readings were stable, typically <0.01 pF. Next, the assemblywas moved into a thermal cycle chamber 126 and the capacitance wasmeasured ten times again. It is preferable to make sure the PCB assemblyis always electrically isolated from earth or machine ground when thecapacitance is being measured. If the heat spreader were connected toground it may not be possible to measure the capacitance. The first tenthermal cycles were −40 C to 60° C. to represent the temperatureextremes that are possible when shipping product. Under computer programcontrol, capacitance readings were made on all twenty components everyfive minutes, continuously. After the ten shipping thermal cycles, thecycle was changed to 10 to 70° C., representing an over stress, poweron/off cycle. FIG. 2 shows a typical bond line response vs. temperaturecycling. During the cooling portion of the cycle, the bond lineincreases, indicating that there is a tensile force on the TIM.Conversely, during the heating portion of the cycle, the bond linedecreases, indicating that there is a compressive force on the TIM.Total peak to valley TIM movement appears to be about 0.18 mm. However,it is believed that additional work would aid in quantifying the gapmovement with greater certainty. The bond line calculation uses thedielectric constant of the TIM which could be changing during thethermal cycle. Air could be moving into or out of the bulk TIM andinterfaces under the tensile and compressive loads during the thermalcycle. In a later experiment, gap motion is measured with no TIM presentand the dielectric constant of air is used.

The gap motion that is suggested by the 0.18 mm peak to valley estimateis a concern when compared to the results of tensile adhesion testing ofindividual components to individual heat spreader plates. These samplesreached peaked tensile loads at about 0.22 mm elongation on a 1.5 mmbond line. For the twenty components that were measured, the range inpeak to valley movement was <0.05 mm to almost 0.4 mm. FIG. 3 shows theresults after almost 500 thermal cycles for a component that had a peakto valley change of <0.05 mm. The bond line is very stable and theamplitude of motion is not changing for a fixed thermal cycle. When thethermal cycle changes from 10-70° C. to 10-90° C., compression increasesfrom 70 to 90° C. as shown in the plot in FIG. 3. FIG. 4 shows photos ofthe heat spreader site and the component after disassembly. The fissuresthat appear in the TIM on the heat spreader side are an indicator ofstructure weakness. Data taken from testing individual components matedto individual heat spreader plates (such individual plates representing,e.g., that portion of the common heat spreader that is immediatelyadjacent the component of interest) allows information characteristic ofvarious physical conditions to be obtained. Such information can behelpful in identifying particular physical conditions, such as TIMdegradation, that can be referenced when the individual component isassociated with a heat spreader common to many components (for example,when monitoring an assembly in-situ during actual operation).

FIG. 5 shows the results of the capacitive bond line response for acomponent that had a peak to valley change of almost 0.4 mm. The bondline starts at about 1.1 mm and increases steadily during thermal cycletesting.

A view of the photos in FIG. 6 helps explain the increasing trend in theFIG. 5 plot. It is obvious that the TIM is moving out of the gap fromthe thermal cycle induced motion. The capacitance is decreasing andwould predict an increasing bond line. However, the decreasingcapacitance is due to about a 25% loss of TIM area as shown in the lowerportion of the heat spreader site. Note how the fillet at the top ismuch larger than the fillet at the bottom in the heat spreader site.

Only eight of the twenty sites (modules) that were monitored showed astable capacitive bond line response throughout the thermal cycletesting. Ten sites showed an increasing capacitive bond line whichequated to >15% loss of TIM area and asymmetrical fillets. The cause ofthe TIM movement is believed to be due to the cycles of compressionduring the heating portion of the cycle and tension during the coolingportion. Additional experiments were defined to quantify absolutemotion, mitigate the TIM movement and compare thermal and power cycleinduced motion.

Phase 2 Experiments

In order to achieve higher accuracy in measuring absolute gap movementwithout the TIM present, two improvements to the Phase 1 experimentswere made that reduced the stray capacitance an order of magnitude (from40 to 4 pF). The improvements are shown in FIGS. 19A, 19B and FIG. 20.Instead of using unshielded wire which picked up stray capacitance fromthe heat spreader 106, coaxial cable was used. The outer conductor 116was an electrical shield that blocked the stray capacitance from theheat spreader 106 to the inner conductor 118 which was used to make thecapacitance measurement. In addition, the copper tape was comprised oftwo layers separated by a dielectric material. The first layer 112 ofcopper tape on the components 102 was connected to electrical shield 116and blocked capacitive coupling from the component and PCB 104. Thesecond layer 110 of copper tape was placed over the shield layer 112 andwas connected to the inner conductor 118 of the coax wire which wasconnected to a scanner channel. The dielectric layer 114 helps preventelectrical shorting between the two copper layers. All the shield wireswere electrically interconnected near the scanner switch box andconnected to the shield of the LCR meter 122. The stability of the straycapacitance vs. temperature was measured to be <0.04 pF in the worstcase.

FIG. 7 shows the air gap changes for all components during a ship shockthermal cycle between −40 C and 60 C. Consistent with the trend in thefirst test, at the cold temperature, the air gap increases as much astwenty (20) microns, and at the hot temperature, it decreases as much aseighty (80) microns. The air gap motion was converted to strain forcorrelation to changes in capacitive bond line of the TIM during thermalcycling and evidence of pumping. The nominal air gap strain was sixpercent, as seen in FIG. 8.

Quantifying the gap motion enabled studies on how to reduce or eliminateTIM movement. Roughening the heat spreader surface has been found toreduce thermal-cycling-induced movement of thermal greases. Straincontrolled, cyclic testing was completed on individual components matedto heat spreader surfaces. Heat spreaders were prepared with N8 and N9surface finishes. (N8 is believed to correspond to an average surfaceroughness of 3.2 microns while N9 is believed to correspond to anaverage roughness of 6.3 microns.) Air gap measurements were made duringa thermal cycle between −40 and 60° C. The heat spreader was removed,TIM was deposited onto the surfaces where the components mated and itwas reattached. Ten cycles of −40 to 60° C. were followed by 350 thermalcycles from 10 to 70° C. As before, stray capacitance was measured forevery component before the TIM was deposited so that it could besubtracted from the capacitance measured with the TIM present.Capacitance measurements were made every five minutes on thirty-twocomponents (modules).

FIG. 9 shows the results for a high strain (8.5%) component site. Thecapacitive bond line shows an upward trend suggesting some TIM pumping.The photos provide evidence of TIM pumping showing asymmetric filletsand fissures in the TIM. The decrease in capacitance equates to a threepercent loss in area.

FIG. 10 shows the results for a nominal strain (5.5%) component site.The capacitive bond line is stable suggesting no significant TIMpumping. The precursors to TIM pumping, asymmetric fillets and fissuresin the TIM are evident in the photos in FIG. 9.

The plot in FIG. 11 shows that there is a mild correlation between theair gap strain and the mean shift in capacitive bond line for thethirty-two components that were tested.

The results for the roughened heat spreader were an improvement comparedto the first test with the non-roughened heat spreader. However, theevidence that there was still a low level of TIM pumping directedfurther study. One objective was to determine if the copper tapeelectrodes 110 caused a different response compared to a component 102without the electrodes. Another objective was to compare thermal cyclingto more realistic power cycling.

Phase 3 Experiments

The disassembly results after phases 1 and 2 showed the presence of theprecursors to pumping and TIM pumping. These results were used to helpinterpret the TIM responses when no copper tape electrodes were presentduring both thermal and power cycling. First, a heat spreader withoutroughening (smooth) was used and, using a power source 128 as shown inFIG. 19A, approximately 500 system power up and down cycles were run.During disassembly, it was observed that ten (10) out of thirty-seven(37) components 102 showed no evidence of TIM pumping as represented inFIG. 12. However, the other twenty-seven components had evidence of TIMpumping: asymmetric fillets (8), fissures (15), both asymmetric filletsand fissures (4) and 5-15% loss of TIM area (10). FIG. 13 shows atypical response with an asymmetrical fillet and a large fissure thatrepresents a loss in TIM area of about 10%. The same node assembly wasprepared for thermal cycle testing out to 350 cycles. Similar TIMmovement occurred in both power cycling (FIG. 13) and thermal cycling(FIG. 14) when a smooth heat spreader was used and no copper tapeelectrodes were present.

Next, the response for a heat spreader with an N9 surface finish wascompared after exposure to 1250 power cycles and more than 1000 thermalcycles, again without any copper tape electrodes. FIGS. 15 and 16 arephotos of typical sites after thermal and power cycling, respectively.In these figures, there is no significant evidence of TIM pumping. Onother components not shown, there are infrequent occurrences of verysmall fissures that do not add up to any quantifiable loss in TIM area.

It is concluded that the copper electrodes on the components do notalter the results of TIM pumping for the smooth heat spreaders. Thebenefit of roughening the heat spreader is also evident when the copperelectrodes are used. However, the precursor evidence to TIM pumping isnearly completely absent on the assembly that was thermal cycled withoutcopper tape electrodes on the components. In contrast, twenty-seven (27)out of thirty-seven (37) components with copper tape electrodes thatwere thermal cycled showed the precursor signs of TIM pumping. In thistest, though, the capacitive bond line was stable on all components.

One last test was performed to quantify the gap motion during powercycle and compared to the range of 100 microns that was previouslymeasured in thermal cycle. Power cycling was performed with and withoutTIM. The agreement in the absolute gap measurements that were made withand without TIM was 12% or less, as shown in FIG. 17. As seen therein,capacitive gap measurements during power cycling are consistent with andwithout TIM.

The measurements were less noisy for the TIM gaps compared to the airgaps because of the 3.45 dielectric constant of the TIM. Consequently,changes in gap during power cycling were quantified using the TIM. Therange in motion was only 10 microns compared to 100 microns that wasmeasured in thermal cycling, as shown in FIG. 18.

Findings based on use of the capacitance measurements as discussed aboveinclude:

-   -   1. Assemblies with a smooth heat spreader experience increasing        capacitive bond line during thermal cycle testing equivalent as        much as 25% of area loss in TIM. TIM pumping occurs with and        without copper tape electrodes during thermal cycling and        without copper electrodes during power cycling. (Extended power        cycling was not done with copper electrodes on the component        surfaces.)    -   2. Assemblies with an N9 heat spreader and copper tape        electrodes have a more stable capacitive bond line and        equivalent area losses in TIM are less than 5%. Precursors to        TIM pumping are evident: fissures and asymmetric fillets    -   3. Assemblies with an N9 heat spreader but without copper tape        electrodes have very few and small fissures after both thermal        and power cycling. No quantifiable area loss in TIM is observed.    -   4. The range of motion in power cycling is a factor of ten (10)        less than in thermal cycling.

The combined benefits of a roughened heat spreader surface and an ordermagnitude less gap motion in power cycling compared to thermal cyclingprovide confidence that thermal performance will be maintained underoperational life.

Monitoring the response of the TIM with capacitance is easy and providestimely insight throughout the development cycle. Confidence can begained on the reliability of the thermal solution well before thermalhardware is available for final system level thermal testing.

FIG. 19C shows a modified assembly similar to that shown in FIG. 19A.The same reference numerals found in FIG. 19A are employed in FIG. 19Cto designate similar elements. In this assembly, the heat spreader 106additionally functions as an electrical shield to reduce the electricalnoise during system power-on. The heat spreader 106 is electricallyconnected to a shield connection 116B. The shield connection 116B iselectrically coupled to the electrical shield 116. A layer of coppertape 110B is applied to the heat spreader and matched to the componentlocation and area. The copper tape 110B is connected to the scannerswitch box 120 and is electrically isolated from the heat spreader by athin layer of dielectric material (not shown). The copper tape 110Baccordingly functions as one of the parallel plates of a capacitor, thesecond plate comprising the copper layer 110. The heat spreader 106,being electrically isolated from the system and earth ground, acts as anelectrical shield and blocks out capacitive coupling from theelectrically active system.

FIG. 21 presents a flow chart 2100 of exemplary method steps, accordingto an aspect of the invention. Processing begins at 2102. In step 2104,appropriate initialization is carried out; for example, initialize theLCR meter under GPM or similar control (also initialize the scanner inone or more embodiments). The meter should be set to the desired testmode (e.g., capacitance as opposed to inductance or resistance) and thedrive voltage, frequency, and time between measurements (e.g., 10seconds, 30 seconds, 50 seconds, or other appropriate value) should beset. Of course, in other instances, a capacitance only meter could beused. In step 2106, identify the channel(s) to be measured in thisinstance for the particular device(s) under test (DUT) (for example,consult a look-up table). Beginning at a suitable channel (e.g., channel1), select the required channel on the scanner switch and begin makingone or more measurements for that channel, in step 2108. As shown atdecision block 2110, in a typical case, multiple measurements are madeon each channel and the mean and standard deviation are determined. Aclip level can be set to reject extraneous data, if desired. Where morereadings on the same channel are required (“Y” branch of block 2110),loop back to 2108. Where no more readings on the same channel arerequired (“N” branch of block 2110), proceed to decision block 2112 anddetermine if there are more channels to be tested. If so (“Y” branch of2112), loop back to 2108 and test for those channels. If not (“N” branchof 2112), proceed to 2113. In step 2113, the pertinent environmentalparameters or other boundary conditions can be varied (deliberately oras may occur in-situ during use); if more variations are to be tested,return to step 2108 as per the “Y” branch; else, proceed to step 2114and output a suitable data file including measurements (capacitance),channel, time when taken, and optionally, pertinent environmentalparameters or other boundary conditions such as temperature, humidity,power dissipated, applied assembly force or pressure, vibration input,and the like.

In step 2116, carry out post processing on the data file to determinethe presence of signatures of anomalies such as TIM pumping or otherbond line degradation. This step can include converting capacitance tothe parameters of interest, such as bond line thickness, thermalconductance, or the like. For example, measure the capacitance of theDUT, subtract out the stray capacitance, and determine g from equation(1). With reference to FIG. 5 (bond line increasing with cycle),exemplary information of interest might include the amplitude of bondline change with temperature, amplitude of total excursion of bond lineover a number of cycles (flat slope desirable but increasing slopeundesirable).

Processing continues at 2118.

As noted at 2108, in addition to or in lieu of post-processing step2116, real time examination for anomalies such as TIM pumping or otherbond line degradation can be carried out at any suitable point duringthe data gathering process (within step 2108 being but one non-limitingexample).

Given the discussion thus far, it will be appreciated that, in generalterms, an exemplary method for monitoring thermal interface material inthe gap between an electronic component and a heat spreader, accordingto an aspect of the invention, includes the step of applying an electricpotential to first and second electrodes on opposite sides of the gap.At least one of the thermal interface material, electronic component andheat spreader is subjected to a changing physical condition such aspressure, temperature cycling and/or power cycling. The electricalcapacitance between the electrodes is monitored during the changingphysical condition. The method can be applied to a plurality ofelectronic components that are mounted to a PCB and use a common heatspreader.

The principles of the invention may be applied to monitoring variousinterfaces between elements, including interfaces comprising bond linesmade of materials having dielectric properties such as thermal interfacematerials as discussed above, adhesives, and other dielectric materialspositioned between two elements. One or more of the elements may be anelectronic component such as a module or microprocessor. Alternatively,it may be useful to monitor interfaces outside the field of electronics.The capacitance between electrodes strategically positioned at suchinterfaces can be monitored by equipment as disclosed herein, in situand in real time, providing information relating to the interfaces andthe material, if any, positioned within the interfaces. In this mannerit may be possible to monitor for possible degradation at interfaces ofload-bearing structures or other structures where changes at interfacesare useful to know but may otherwise be difficult to determine visuallyor by other means. Non-limiting examples include adhesive bonds in avariety of contexts, such as automotive, aerospace, and construction andcivil engineering applications (including initial assembly integrity andin-situ monitoring over time).

Furthermore, given the discussion thus far, it will be appreciated that,in general terms, an exemplary assembly for testing thermal interfaces,according to an aspect of the invention, includes a PCB, a plurality ofelectronic components mounted to the PCB, heat spreader, a firstelectrode associated with the heat spreader, a plurality of secondelectrodes associated, respectively, with the electronic components, anda device for monitoring the capacitances between the first and secondelectrodes. The heat spreader preferably comprises the first electrode.Thermal interface material may be positioned between the first andsecond electrodes.

The principles of the invention can be applied to constructing anassembly for either testing or long term purposes. An example of aconstruction method that allows external monitoring of a completedassembly includes providing a plurality of components, thermal interfacematerials, electrode plates and conductors, and assembling theseelements to form an integral structure wherein a plurality of interfacesare provided between components, the electrode plates are positionedwithin the interfaces, the interface material is positioned between theelectrodes, and the conductors are electrically connected to theelectrode plates and externally accessible on the integral structure.The method may further include connecting an apparatus for measuringelectrical capacitance and connecting the apparatus to the conductors.The apparatus does not necessarily need to cause the display of units ofelectrical capacitance as it may be sufficient only to know that theelectrical capacitance between electrode plates has changed or that theelectrical capacitance measured between one set of electrode plates ismaterially different from one or more other sets of plates. Electrodeplates can be incorporated within the integral structure at locationsthat may be deemed critical, most subject to failure, most difficult orimpossible to visually inspect, and/or at random. The integral structureconstructed in this manner can be monitored at the site of the structureor at a remote site. The interfaces can be monitored simultaneously orindividually.

Thus, aspects of the invention include a test method, a method ofdesigning a thermal interface based on such testing, a computer programproduct which facilitates such testing, a test set-up or apparatus, amethod of construction, and a test assembly wherein the components to betested are mounted in connection with the test set-up or apparatus.

Exemplary System and Article of Manufacture Details

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

One or more embodiments of the invention, or elements thereof, can beimplemented in the form of an apparatus including a memory and at leastone processor that is coupled to the memory and operative to performexemplary method steps.

One or more embodiments can make use of software running on a generalpurpose computer or workstation. With reference to FIG. 22, such animplementation might employ, for example, a processor 2202, a memory2204, and an input/output interface formed, for example, by a display2206 and a keyboard 2208. The term “processor” as used herein isintended to include any processing device, such as, for example, onethat includes a CPU (central processing unit) and/or other forms ofprocessing circuitry. Further, the term “processor” may refer to morethan one individual processor. The term “memory” is intended to includememory associated with a processor or CPU, such as, for example, RAM(random access memory), ROM (read only memory), a fixed memory device(for example, hard drive), a removable memory device (for example,diskette), a flash memory and the like. In addition, the phrase“input/output interface” as used herein, is intended to include, forexample, one or more mechanisms for inputting data to the processingunit (for example, mouse), and one or more mechanisms for providingresults associated with the processing unit (for example, printer). Theprocessor 2202, memory 2204, and input/output interface such as display2206 and keyboard 2208 can be interconnected, for example, via bus 2210as part of a data processing unit 2212. Suitable interconnections, forexample via bus 2210, can also be provided to a network interface 2214,such as a network card, which can be provided to interface with acomputer network, and to a media interface 2216, such as a diskette orCD-ROM drive, which can be provided to interface with media 2218.

Accordingly, computer software including instructions or code forperforming the methodologies of the invention, as described herein, maybe stored in one or more of the associated memory devices (for example,ROM, fixed or removable memory) and, when ready to be utilized, loadedin part or in whole (for example, into RAM) and implemented by a CPU.Such software could include, but is not limited to, firmware, residentsoftware, microcode, and the like.

A data processing system suitable for storing and/or executing programcode will include at least one processor 2202 coupled directly orindirectly to memory elements 2204 through a system bus 2210. The memoryelements can include local memory employed during actual implementationof the program code, bulk storage, and cache memories which providetemporary storage of at least some program code in order to reduce thenumber of times code must be retrieved from bulk storage duringimplementation.

Input/output or I/O devices (including but not limited to keyboards2208, displays 2206, pointing devices, and the like) can be coupled tothe system either directly (such as via bus 2210) or through interveningI/O controllers (omitted for clarity).

Network adapters such as network interface 2214 may also be coupled tothe system to enable the data processing system to become coupled toother data processing systems or remote printers or storage devicesthrough intervening private or public networks. Modems, cable modem andEthernet cards are just a few of the currently available types ofnetwork adapters.

As used herein, including the claims, a “server” includes a physicaldata processing system (for example, system 2212 as shown in FIG. 22)running a server program. It will be understood that such a physicalserver may or may not include a display and keyboard.

As noted, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon. Anycombination of one or more computer readable medium(s) may be utilized.The computer readable medium may be a computer readable signal medium ora computer readable storage medium. A computer readable storage mediummay be, for example, but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,or device, or any suitable combination of the foregoing. Media block2218 is a non-limiting example. More specific examples (a non-exhaustivelist) of the computer readable storage medium would include thefollowing: an electrical connection having one or more wires, a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), an optical fiber, a portable compact disc read-onlymemory (CD-ROM), an optical storage device, a magnetic storage device,or any suitable combination of the foregoing. In the context of thisdocument, a computer readable storage medium may be any tangible mediumthat can contain, or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language, BASICprogramming language, or similar programming languages. The program codemay execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

It should be noted that any of the methods described herein can includean additional step of providing a system comprising distinct softwaremodules embodied on a computer readable storage medium; the modules caninclude, for example, any or all of the elements depicted in the blockdiagrams and/or described herein; by way of example and not limitation,an initialization module, a module to cycle through the test points andparameters, an output module to generate the output file, apost-processing module to reduce the data and search for anomalies, andthe like. The method steps can then be carried out using the distinctsoftware modules and/or sub-modules of the system, as described above,executing on one or more hardware processors 2202. Further, a computerprogram product can include a computer-readable storage medium with codeadapted to be implemented to carry out one or more method stepsdescribed herein, including the provision of the system with thedistinct software modules.

In some instances, changing the external temperature induces motion whencomponents have different coefficients of thermal expansion; in additionor as an alternative, a mechanical force can be applied to the heatspreader, to cause a local deformation, and the capacitance response canbe monitored close to the point of mechanical perturbation. Theaforementioned module to cycle through the test points and parameterscould be used to control changes in temperature and/or forceapplication.

In any case, it should be understood that the components illustratedherein may be implemented in various forms of hardware, software, orcombinations thereof; for example, application specific integratedcircuit(s) (ASICS), functional circuitry, one or more appropriatelyprogrammed general purpose digital computers with associated memory, andthe like. Given the teachings of the invention provided herein, one ofordinary skill in the related art will be able to contemplate otherimplementations of the components of the invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method of constructing an assembly capable ofbeing monitored for physical changes, comprising: providing a pluralityof components; providing interface material; providing a plurality ofelectrode plates; providing a plurality of conductors, and assemblingthe components, interface material, electrode plates and conductors toform an integral structure wherein a plurality of interfaces areprovided between the components, the electrode plates are positionedwithin the interfaces, the interface material is positioned between theelectrode plates, and the conductors are electrically connected to theelectrode plates and externally accessible on the integral structuresuch that the electrical capacitance between electrode plates in aplurality of the interfaces can be electronically monitored byconnecting an apparatus for detecting electrical capacitance to theconductors and applying electrical power to the electrode plates.
 2. Themethod of claim 1, further including connecting an apparatus formeasuring electrical capacitance to the conductors.
 3. The method ofclaim 2 wherein at least one of the components comprises one of theelectrode plates.
 4. The method of claim 2 wherein the componentsinclude electronic components, a printed circuit board, and a heatspreader, further including assembling the components such that theinterfaces are provided between the electronic components and the heatspreader.
 5. The method of claim 4, further including providing aconductor in adjoining relation to the heat spreader, the conductorcomprising one of the electrode plates.
 6. The method of claim 5,further including configuring the heat spreader to function as anelectrical shield.